Thin Film Gas Sensor Configuration

ABSTRACT

A gas sensor for sensing a gas stream constituent includes, mounted a substrate, a first gas-sensing element capable of sensing the constituent in a first concentration range, a reference element insensitive to the constituent and having electrical properties congruent with the first gas-sensing element, a heating element substantially circumscribing the first gas-sensing element and the reference element, a temperature-sensing element circumscribing the first gas-sensing element and the reference element, and a second gas-sensing element capable of sensing the constituent in a second concentration range. The first gas-sensing element and the reference element are preferably metal-gated metal-oxide semiconductor (MOS) solid-state devices. The gas sensor is particularly configured to sense hydrogen concentration in a gas stream.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.11/046,370, filed on Jan. 27, 2005. The '370 application was related toand claimed priority benefits from U.S. Provisional Patent ApplicationSer. No. 60/540,020, filed on Jan. 27, 2004. The '307 nonprovisionalapplication and the '020 provisional application are each herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to sensors for detecting the presence of aconstituent in a gaseous stream. More particularly, the presentinvention relates to a hydrogen gas sensor configuration having firstand second hydrogen sensing elements, a reference element, a heatingelement, and a temperature-sensing element.

BACKGROUND OF THE INVENTION

In gas sensor applications, the arrangement of the sensor elements ontheir underlying substrate should exhibit certain attributes to improveor optimize their performance. In particular, it is desirable tophysically arrange and integrate the sensing and operational elements ofthe sensor so that the components are maintained at essentially the sametemperature. In practice, the sensor elements should be arranged so asto minimize the substrate area occupied by the elements, therebyreducing or minimizing thermal convection///and conduction losses amongthe sensor elements. Secondarily, minimizing the occupied substrate areaalso reduces the amount of substrate material required to fabricate thesensor, and thus reduces fabricating costs.

Conventional, prior art solutions, such as those developed at SandiaNational Laboratories (see R. Thomas and R. Hughes, “Sensors forDetecting Molecular Hydrogen Based on PD Metal Alloys”, J. Electrochem.Soc., Vol. 144, No. 9, September 1997; and U.S. Pat. No. 5,279,795),involve the interlacing of the sensing and operational elements of thesensor. Such conventional solutions employ a geometry that deploys thesensor elements over a significantly larger area than necessary, thusrendering the design less effective in terms of thermal layout, in thatthe sensing element(s) of the sensor (capacitive metal-on-silicon (MOS)elements in the case of the Sandia design) do not occupy a common,uniform thermal environment. The Sandia design also has greater thanoptimal manufacturing costs due to the interlaced design renderingunused significant portions of the underlying substrate material. TheSandia and similar prior art designs did not seek to optimize thethermal environment of the sensor assembly. Nor did the Sandia orsimilar prior art designs seek to optimize the mechanical compactness ofthe sensor assembly.

Although conventional, prior art solutions had some thermal integrationof the heating element, the temperature sensor and the gas sensor, thegeometry was such that these elements could become flow sensitive. Flowsensitivity refers to the effect that the flow rate of the gas stream tobe measured can have in conducting heat from the element(s), therebylowering their temperature and requiring additional electrical power torestore the temperature of the element(s) to their original and desiredlevel. Sandia-type sensor designs included an additional capacitive(MOS) sensor, which was located outside of the portion of the assemblyhaving controllable and uniform thermal properties. Moreover, suchconventional, prior art designs had considerable wasted space on theunderlying silicon die or substrate on which the sensor elements werearranged, which would multiply (approximately triple) the manufacturingcosts and heat loss from the sensor elements to the externalenvironment.

In the present gas sensor assembly, the sensing and control elements aremounted on the underlying substrate and operated so as to maintainthermal integrity and prevent heat loss. In particular, the areaoccupied by the sensor elements is minimized or conserved to minimize orreduce thermal losses to convection/conduction///of heat to and from thesensor components. The present gas sensor assembly is also configuredfor compactness to minimize or reduce manufacturing and parts costs bymaximizing or increasing the number of sensor elements mounted on asubstrate.

SUMMARY OF THE INVENTION

One or more of the foregoing shortcomings of conventional, prior art gassensors is overcome by the present gas sensor, which integrates fourthin-film elements in a geometric configuration that conserves and/oroptimizes the area occupied on a die substrate, while reducing and/orminimizing thermal heat losses.

The present gas sensor for sensing a gas stream constituent comprises:

-   -   (a) a thermally conductive, electrically insulative substrate;    -   (b) a first gas-sensing element mounted on the substrate, the        first gas-sensing element capable of sensing the constituent in        a first concentration range;    -   (c) a reference element mounted on the substrate, the reference        element and the first gas-sensing element having congruent        electrical properties, the reference element insensitive to the        constituent;    -   (d) a heating element mounted on the substrate and substantially        circumscribing the first gas-sensing element and the reference        element;    -   (e) a temperature-sensing element mounted on the substrate and        substantially circumscribing the first gas-sensing element and        the reference element;    -   (f) a second gas-sensing element mounted on the substrate        capable of sensing the constituent in a second concentration        range.

In a preferred embodiment of the present gas sensor, each of the firstgas-sensing element and the reference element comprises a materialhaving electrical properties that change upon exposure to the gas streamconstituent. The temperature-sensing element preferably substantiallycircumscribes the heating element. The second gas-sensing elementpreferably substantially circumscribes the first gas-sensing element andthe reference element. The second gas-sensing element preferablysubstantially circumscribes the temperature-sensing element.

In a preferred embodiment of the present gas sensor, each of the firstgas-sensing element and the reference element is a metal-gatedmetal-oxide semiconductor (MOS) solid-state device. The MOS device cancomprise a MOS capacitor. The MOS device can also comprise a MOStransistor. The metal gate of the first gas-sensing MOS devicepreferably comprises a metal selected from the group consisting ofpalladium and a palladium alloy. The palladium alloy is preferablyselected from the group consisting of palladium/nickel, palladium/goldand palladium/silver. The metal gate of the reference element MOS devicepreferably comprises a metal that is inert with respect to the gasstream constituent. The preferred inert metal is gold. The metal gate ofthe reference element MOS device can also comprise a passivated metalthat is non-inert with respect to the gas stream constituent. Thenon-inert metal is preferably passivated by application of an inertcoating material, such as glass or an inert polymeric material.

In a preferred embodiment of the present gas sensor, the substratecomprises a silicon-containing material. The heating element ispreferably a resistive heating element. The temperature-sensing elementpreferably comprises a material having a stable temperature coefficientof resistance, most preferably nickel. The second gas-sensing element ispreferably a catalytic metal resistor, most preferably apalladium/nickel alloy.

The present gas sensor is particularly configured to sense hydrogenconcentration in a gas stream The first gas-sensing element senseshydrogen in a first concentration range of from 10⁻⁶ Torr to 10 Torr.The second gas-sensing element senses hydrogen in a second concentrationrange of greater than 1 Torr.

A method of sensing a gas stream constituent comprises:

-   -   (a) sensing the constituent in a first concentration range by        measuring a voltage difference between a first gas-sensing        element mounted on a substrate and a reference element mounted        on the substrate, the reference element and the first        gas-sensing element having congruent electrical properties, the        reference element insensitive to the constituent; and    -   (b) sensing the constituent in a second concentration range by        measuring a change in electrical properties of a second        gas-sensing element mounted on the substrate.

A preferred embodiment of the sensing method further comprises:

-   -   (c) maintaining the first gas-sensing element and the reference        element in a uniform temperature environment.        The uniform temperature environment is preferably maintained        using a heating element that is responsive to a        temperature-sensing element, the heating element substantially        circumscribing the first gas-sensing element and the reference        element, and the temperature-sensing element substantially        circumscribing the first gas-sensing element, the reference        element and the heating element.

A method of fabricating a gas sensor for sensing a gas streamconstituent, the fabricating method comprises:

-   -   (a) mounting a first gas-sensing element on a thermally        conductive, electrically insulative substrate, the first        gas-sensing element capable of sensing the constituent in a        first concentration range;    -   (b) mounting a reference element on the substrate, the reference        element and the first gas-sensing element having congruent        electrical properties, the reference element insensitive to the        constituent;    -   (c) mounting a heating element on the substrate such that the        heating element substantially circumscribes the first        gas-sensing element and the reference element;    -   (d) mounting a temperature-sensing element on the substrate such        that the temperature-sensing element substantially circumscribes        the first gas-sensing element and the reference element; and    -   (e) mounting a second gas-sensing element on the substrate, the        second gas-sensing element capable of sensing the constituent in        a second concentration range.

In the preferred fabricating method, each of the first gas-sensingelement and the reference element comprises a material having electricalproperties that change upon exposure to the gas stream constituent. Thetemperature-sensing element preferably substantially circumscribes theheating element. The second gas-sensing element preferably substantiallycircumscribes the first gas-sensing element and the reference element.The second gas-sensing element more preferably substantiallycircumscribes the temperature-sensing element. Each of the firstgas-sensing element and the reference element is preferably ametal-gated metal-oxide semiconductor (MOS) solid-state device. The MOSdevice can comprise a MOS capacitor and can also comprise a MOStransistor.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a schematic diagram showing the present thin film gas sensorin plan view.

FIG. 2 is an enlarged view of the proximal end of a flexible circuitthat incorporates the gas sensor of FIG. 1.

FIG. 3 shows in plan view the gas sensor of FIG. 1 incorporated at thedistal end of a flexible circuit having pin connections at its proximalend, which extend from the flexible for mounting on a circuit board.

FIG. 4 is a schematic circuit diagram identifying the pin connectionsfor each of the gas sensor elements illustrated in FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

The principal improvement achieved by the present thin film gas sensordesign is that of employing a compact, thermally efficient design thathas geometrical symmetry and surrounds that geometry with heating andsensing elements. The geometry is regular to minimize or reduce unuseddie surface area. The compact configuration minimizes or reducessensitivity to temperature differences across the surface area of thesensor, both by virtue of the integrated geometry and the compactgeometry.

Turning to FIG. 1, a thin film gas sensor 100 is capable of sensing aconstituent (hydrogen in the device depicted in the figures anddescribed in detail below) in a gas stream directed across gas sensor100. Sensor 100 includes a thermally conductive, electrically insulativesubstrate 150. Substrate 150 preferably comprises a silicon-containingmaterial. A first gas-sensing element 160 is mounted on substrate 150.The first gas-sensing element capable of sensing the constituent in afirst concentration range.

As further shown in FIG. 1, a reference element 170 is mounted on thesubstrate. While first gas-sensing element 160 is sensitive to hydrogen,reference element 170 is insensitive to hydrogen. However, referenceelement 170 and first gas-sensing element 160 have congruent electricalproperties (that is the electrical properties of reference element 170,if rendered sensitive to hydrogen, would exhibit changes in voltageand/or resistance corresponding to the changes in voltage and/orresistance exhibited by first gas-sensing element 160).

A heating element 180 is mounted on substrate 150, and substantiallycircumscribes first gas-sensing element 160 and reference element 170,as shown in FIG. 1. A temperature-sensing element 190 is also mounted onsubstrate 150 and substantially circumscribes first gas-sensing element160 and reference element 170. In the illustrated configuration,temperature-sensing element 190 also substantially circumscribes heatingelement 180.

A second gas-sensing element 210 is also mounted on substrate 150. Inthe illustrated configuration, second gas-sensing element 210substantially circumscribes first gas-sensing element 160 and referenceelement 170, and also substantially circumscribes temperature-sensingelement 190. Second gas-sensing element 210 shown in FIG. 1 is acatalytic metal resistor, preferably formed from a palladium/nickelalloy, and is capable of sensing hydrogen in a second concentrationrange.

First gas-sensing element 160 and reference element 170 are preferablymetal-on-silicon (MOS) capacitors. Such MOS devices are not restrictedto a capacitive form, however, but could be implemented in p-n-ptransistor, field-effect transistor (FET) or diode configurations aswell.

In gas sensor 100 illustrated in FIG. 1, each of first gas-sensingelement 160 and reference element 170 is a metal-gated metal-oxidesemiconductor (MOS) solid-state capacitive device. The metal gate offirst gas-sensing element 160 is preferably formed frompalladium/nickel. The metal gate of the reference element 170 ispreferably formed from gold. Heating element 180 shown in FIG. 1 is aresistive heating element. Temperature-sensing element 190 shown in FIG.1 is preferably formed from nickel. Second gas-sensing element 210 is acatalytic metal resistor, preferably formed from a palladium/nickelalloy.

As shown in FIG. 1, first gas-sensing element 160 has a terminal 160 aformed therein. Reference element 170 has a terminal 170 a formedtherein. Heating element 180 has a pair of terminals 180 a, 180 b formedtherein at opposite ends of its trace on substrate 150.Temperature-sensing element 190 has two pairs of terminals 190 a, 190 band 190 c, 190 d formed therein at opposite ends, respectively, of itstrace on substrate 150. Second gas-sensing element 210 also has twopairs of terminals 210 a, 210 b and 210 c, 290 d formed therein atopposite ends, respectively, of its trace on substrate 150.

FIG. 2 is an enlarged view of the proximal end of a flexible circuit(shown as flex circuit 280 in FIG. 3) that incorporates gas sensor 100of FIG. 1. FIG. 2 shows in detail the electrical connections between gassensor 100 and the flex circuit. The flex circuit has a plurality ofcopper traces disposed on its upper surface. As illustrated in FIG. 3and described in the text below accompanying FIG. 3, the copper tracesat the distal end of the flex circuit extend to and are electricallyconnected to pin connections, located at the proximal end of flexcircuit, for mounting on a circuit board (not shown).

As shown in FIG. 2, sensor 100 includes substrate 150, on which aremounted first gas-sensing element 160, reference element 170, heatingelement 180, temperature-sensing element 190 and second gas-sensingelement. As further shown in FIG. 2, bond wire 235 connects firstgas-sensing element 160 to copper trace 255. Bond wire 234 connectsreference element 170 to copper trace 254. Bond wires 232, 233 connectheating element 180 to copper traces 252, 253, respectively. Two pairsof bond wires 240, 242 and 248, 250 connect temperature-sensing element190 to copper traces 260, 262 and 268, 270, respectively. Similarly, twopairs of bond wires 236, 238 and 244, 246 connect second gas-sensingelement 210 to copper traces 256, 258 and 264, 266, respectively.Substrate 150 is electrically connected to copper trace 263.

FIG. 3 shows gas sensor 100 incorporated at the distal end of flexcircuit 280, which has pin connections PIN 1 through PIN 16 at itsproximal end for mounting on a circuit board (not shown in FIG. 3). Forsimplicity and ease of interpretation, FIG. 3 shows only the connectionsof the copper traces to the pin connections. Thus, although the wireconnections at the proximal end of flex circuit 280 are not shown,persons skilled in the technology involved here will recognize that theproximal end wire connections omitted from FIG. 3 can be readilyconfigured using ordinary circuit design techniques.

As shown in FIG. 3, the proximal end of trace 252 terminates at aterminal other than a pin connection, from which it is electricallyconnected by a wire (not shown) to another terminal or pin. The proximalend of trace 253 terminates at PIN 8. The proximal end of trace 254terminates at PIN 11. The proximal end of trace 255 terminates at PIN 7.The proximal end of trace 256, 258 terminate at adjacent terminals otherthan a pin connection, from which they are electrically connected bywires (not shown) to other terminal(s) or pin(s). The proximal end oftrace 260 terminates at PIN 12. The proximal end of trace 262 terminatesat PIN 13. The proximal end of trace 263 terminates at an adjacentterminal other than a pin connection, from which it is electricallyconnected by a wire (not shown) to another terminal or pin. The proximalend of trace 264 terminates at PIN 14. The proximal end of trace 266terminates at PIN 15. The proximal end of trace 268 terminates at PIN 5.The proximal end of trace 270 terminates at PIN 6. PIN 2, PIN 3 and PIN4 are electrically connected by copper traces to terminal other than apin connection, from which it is electrically connected by a wire (notshown) to other terminal(s) or pin(s). PIN 1 and PIN 16 are notconnected to traces emanating from gas sensor 100. Each of PIN 1 throughPIN 16 extends downwardly through flex circuit and are insertable intoaligned mounting holes in a circuit board (not shown), which containsthe downstream processing and control circuitry to which the signalsfrom flex circuit 280 are directed.

FIG. 4 is a schematic circuit diagram identifying the pin connectionsfor each of the gas sensor elements. Gas-sensing capacitor 160 isconnected at one end to PIN 11 and at its other end to PIN 10. Referencecapacitor 170 is connected at one end to PIN 7 and at its other end toPIN 10. Heater resistor 180 is connected at one end to PIN 1 and PIN 8and at its other end to PIN 9. Temperature-sensing resistor 190 isconnected at one end to PIN 14 and PIN 15 and at its other end to PIN 12and PIN 13. Gas-sensing resistor 210 is connected at one end to PIN 5and PIN 6 and at its other end to PIN 3 and PIN 4.

The integrated die sensor assembly has been tested and has been shown tomaintain favorable functional isolation among the sensing elements,exhibits reduced flow sensitivity, is thermally responsive, and iseasily manufactured.

The advantages are the present thin film gas sensor include:

-   -   (a) rapid temperature response due the minimization of die area        and thereby mass;    -   (b) reduced sensitivity to gas stream flow;    -   (c) effective thermal coupling among the sensor and control        elements;    -   (d) ease of manufacture; and    -   (e) ease of packaging onto either standard dual in-line packages        or flex harness.

Although the present device has been implemented in its preferredembodiment to sense hydrogen, persons skilled in the technology involvedhere will recognize that one or more aspects of the present device couldbe implemented or readily modified to sense and/or detect the presenceand/or amount of constituents in fluid streams generally, including gasstreams containing hydrogen and/or other than hydrogen, liquid streams,liquid streams containing entrained gas(es) and/or solid(s), gas streamscontaining entrained liquid(s) and/or solid(s). Moreover, aspects of thepresent device could be implemented or readily modified to sense and/ordetect the presence and/or amount of fluid constituents residing in thepores and/or lattice structure of solids

While particular steps, elements, embodiments and applications of thepresent invention have been shown and described, it will be understood,of course, that the invention is not limited thereto since modificationscan be made by those skilled in the art, particularly in light of theforegoing teachings.

1. A gas sensor for sensing a gas stream constituent, the gas sensorcomprising: (a) a thermally conductive, electrically insulativesubstrate; (b) a first gas-sensing element mounted on said substrate,said first gas-sensing element capable of sensing said constituent in afirst concentration range; (c) a reference element mounted on saidsubstrate, said reference element and said first gas-sensing elementhaving congruent electrical properties, said reference elementinsensitive to said constituent; (d) a heating element mounted on saidsubstrate such that a uniform temperature environment is effected aroundsaid first gas-sensing element and said reference element; (e) atemperature-sensing element mounted on said substrate and in thermalconnection with said first gas-sensing element and said referenceelement; (f) a second gas-sensing element mounted on said substratecapable of sensing said constituent in a second concentration range suchthat said heating element imparts at least some heat to said secondgas-sensing element; wherein said temperature-sensing element isthermally connected to said heating element.
 2. The gas sensor of claim1 wherein said second gas-sensing element is mounted on said substratesuch that a uniform temperature environment is effected around saidfirst gas-sensing element, said second gas-sensing element and saidreference element.
 3. A method of sensing a gas stream constituentcomprising: (a) sensing said constituent in a first concentration rangeby measuring a voltage difference between a first gas-sensing elementmounted on a substrate and a reference element mounted on saidsubstrate, said reference element and said first gas-sensing elementhaving congruent electrical properties, said reference elementinsensitive to said constituent; (b) sensing said constituent in asecond concentration range by measuring a change in electricalproperties of a second gas-sensing element mounted on said substrate;(c) maintaining said first gas-sensing element, said second gas-sensingelement, and said reference element in a uniform temperatureenvironment, wherein said uniform temperature environment is effected bya heating element that is responsive to a temperature-sensing element.4. A method of fabricating a gas sensor for sensing a gas streamconstituent, the fabricating method comprising: (a) mounting a firstgas-sensing element on a thermally conductive, electrically insulativesubstrate, said first gas-sensing element capable of sensing saidconstituent in a first concentration range; (b) mounting a referenceelement on said substrate, said reference element and said firstgas-sensing element having congruent electrical properties, saidreference element insensitive to said constituent; (c) mounting aheating element on said substrate such that a uniform temperatureenvironment is effected around said first gas-sensing element and saidreference element; (d) mounting a temperature-sensing element on saidsubstrate such that said temperature-sensing element is in thermalconnection with said first gas-sensing element and said referenceelement; (e) mounting a second gas-sensing element on said substrate,said second gas-sensing element capable of sensing said constituent in asecond concentration range, such that at least some heat from saidheating element is imparted to said second gas-sensing element; whereinsaid temperature-sensing element is thermally connected to said heatingelement.
 5. The fabricating method of claim 4 wherein said secondgas-sensing element is mounted on said substrate such that a uniformtemperature environment is effected around said first gas-sensingelement, said second gas-sensing element and said reference element.